Electrophoresis is a technique for separating the charged components of a single or multi-component solution by utilizing differences in the rates of migrations of the charged components in an electric field. The mobility of each charged component, which is equal to the migration rate divided by the field strength, is roughly proportional to its charge to mass ratio. If electrophoresis is performed in a porous medium such as a gel, where molecular sieving takes place, the mobility is also a function of size.
Electrophoresis is a useful tool for separating biological materials such as proteins and nucleic acids. It is particularly useful for nucleic acid sequencing.
Traditionally, electrophoretic separation of biological materials was performed using slabs of gelled materials such as crosslinked polyacrylamide. More recently, electrophoretic separations using very small bore capillary tubes have become popular. This technique requires only very small amounts of samples, and provides high resolution and faster throughput per individual sample than gels.
Capillary electrophoresis is generally performed using fused silica capillary tubes. The tubes may have inner diameters in the range of about 20 to 1000 microns, although typically the inner diameters are no greater than about 100 microns. The tube is coated on its outer surface along its length with an opaque polyimide coating to prevent breakage. In some cases, the separation is performed by filling the capillary tube with only a buffer solution, while in other cases a viscous polymer or gel is added to improve the separation. In general, a sample is introduced into the inlet end of the capillary tube and an electric field applied. Under the influence of the electric field, the sample separates, causing the individual components to migrate down the length of the capillary tube. At the outlet end of the capillary tube, a small region of the opaque polyimide coating is removed to form an optical detection region. The individual components are then detected using, for example, fluorescence or ultraviolet absorbance. Once the polyimide coating has been removed, it is necessary to protect the detection region carefully because the tube is very fragile in the absence of the coating.
Capillary electrophoresis instruments generally employ a single capillary tube and run multiple samples in series. Such an arrangement is not conducive to high throughput screening and nucleic acid sequencing, which would require arrays of capillary tubes for simultaneous analysis of many samples. Such arrays require a means for aligning the tubes precisely and reproducibly so that the inlet and outlet end of each tube can be readily associated with each other, an objective made difficult by the small size and flexibility of the individual tubes. While theoretically each tube could be aligned individually, such an approach would be tedious and time-consuming.